The use of powder metallurgy (P/M) to produce steel main bearing caps for passenger vehicle engines has grown from zero to well over 70 million components in service. The material being replaced is cast iron, usually of the type commonly known as “Ductile Cast Iron” (DCI). There are many commercial and technical advantages to using the P/M process, including elimination of many costly machining steps, forming of unique shapes and geometries during the molding (powder compaction) stage, and material versatility. The large majority of engines used in automotive vehicles fall in a power density ratio (power to engine-size ratio) that places stresses on the main bearing caps that can be accommodated by the P/M steel's inherent material strength. However, there are some special purpose high performance engines that are used for special road cars including racing that go beyond the normal power density ratios. In these special cases, the main bearing cap's performance safety factor is reduced from the preferred minimum of 1.5 to a level approaching 1.0. The 1.0 safety factor means that the component would only just survive the maximum rated engine performance in the long term.
In such cases, it is appropriate to enhance the performance (strength under cyclic fatigue conditions) of the main bearing cap to provide a comfortable safety margin.
The P/M steel materials used for main bearing caps can be strengthened by conventional means such as heat-treating of the material (quench-hardening). In this case, the material is inevitably much harder, and is therefore resistant to the machining operations that are required after the component is installed in the engine cylinder block.
A virtually unique property of metals processed by powder metallurgy is the capability to vary the density, which is the mass per unit volume of the material. This property naturally develops during the P/M manufacturing process that is well known to those versed in the art. Briefly, this consists of compacting the selected powder mix, under high pressure, in specifically designed tooling, into a shape known as a “pre-form”, which is then thermally treated by a process known as “sintering”, which causes the powder particles to fuse together, thereby developing mechanical strength.
It is also well known to those versed in the art that the physical and mechanical properties of the P/M metal increase as the density of the metal increases.
Therefore, to increase the strength of a P/M steel main bearing cap without prejudicing the ease of machining (machinability), it is appropriate to raise the density of the compact. This can normally be achieved by raising the powder compaction pressure, but this option is limited by the strength of the compaction tooling. Alternatively, the design can be simplified to enable more robust tooling to be designed that can withstand higher compaction pressure, but this invariably leads to additional costly machining operations, which may render the product non-viable commercially.
A special feature of metals that are at less than full density is the ability to locally densify the surface by application of mechanical pressure. This can be achieved in several ways, for example by rolling a hard roller over the surface (burnishing), or by localized hammering (peening). Such local densification processes are known to those versed in the art. These processes, when correctly applied, may also result in favorable “residual compressive surface stresses” that can extend the operational life of the product under cyclic fatigue conditions.
This invention teaches a method of incorporating these principles in a new way to enhance the performance of powder metal mechanical components, and in particular, a powder metal steel main bearing cap to meet the demands of modem high performance car engines.
There are three principal mechanical failure modes associated with high performance engine main bearing caps, namely fatigue cracking through the bolted face (FIG. 1a), fatigue cracking through the inner bolt hole (FIG. 1b), and side bolt-hole thread failure (FIG. 1c).
A research program was initiated at the inventor's company to determine if and how the strength of the main bearing cap could be raised by application of surface densification to each of these critical areas. This required extensive processing development work, plus many long term fatigue tests on both test pieces and on actual main bearing caps that are in current production.